Save source location information in types

[byorgey]

We save source spans in the AST representing terms, but we don't save any source location information for types; but we should!

One place this recently came up is #2340 , where it's possible to have an error message referring to two different type variables with the same name, where one shadows the other. It could be nice to have source location info in order to disambiguate them in the error message.

I'm not yet sure whether this means saving source location info in every single type and threading it through everywhere, or whether we should have two separate types: one representing an abstract type, and one representing concrete parsed syntax for a type with embedded source info.